Process for the removal of scale from very scaly metal surfaces



United States Patent 3,506,487 PROCESS FOR THE REMOVAL OF SCALE FROM VERY SCALY METAL SURFACES Arnold Leuz, Cologne-Stammheim, and Walter Rogler, Ranzel, Troisdorf, Germany, assignors to Dynamit Nobel Akfiengesellschaft, Troisdorf, Germany, a corporation of Germany No Drawing. Filed Dec. 11, 1967, Ser. No. 689,280 Claims priority, appliclatitsnli Gar-many, Dec. 13, 1966,

Int. Cl. B08b 7/ 02; C23g 1/28 US. Cl. 134-2 4 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a process for removal of scale and rust from metallic surfaces and more particularly relates to a process for removal of scale and rust from metallic surfaces by combination of chemical and mechanical means.

It is known that metal articles whose surfaces are coated with scale or rust can be cleaned by immersion thereof into a caustic soda bath and/or molten caustic potash bath having a sodium hydride content of 0.3 to 5%, and that thereafter the reduced scale or rust can be entirely removed by electrical treatment in an alkal ne bath, such treatment being first anodic and then, by re- Nersal of the polarity, cathodic. However, with this process, it is not possible to satisfactorily clean very scaly objects with only a single caustic immersion and a single anodic and cathodic treatment. Rather, the two step cleaning process has to be repeated several times. This of course makes the process uneconomical and unsuitable for commercial applications.

It is furthermore known in the prior art to remove heavy coatings of scale or rust by mechanical treatment of unetched metal surfaces, i.e., no prior treatment with caustic. For example, unetched metal wire surfaces can be cleaned by means of sandblasting or by bending. However, this type of treatment often damages the metal surface to such an extent that this procedure cannot be applied to a large number of materials. Another disadvantage of mechanical cleaning of this kind is that coiled materials, such as reels of wire or spiral coils have to be unwound and rewound, which is difiicult in the case of heavy gauge wire. Further the use of mechanical means only often has deleterious effects on the properties and resultant qualities of the metal article,

'It is an object of the present invention to provide an improved process for descaling and derusting metal surfaces.

Another object of the invention is to provide an improved process for descaling and derusting metal surfaces which can be simply and economically effected.

Still another object of the invention is to provide an improved process for descaling and derusting metal surfaces which does not disadvantageously affect the desirable properties of the metal articles.

Other objects of the invention will become apparent Patented Apr. 14, 1970 from the following detailed description of the invention.

The above objects may be accomplished by immersing the article to be descaled into a molten alkali bath which contains from 0.3 to 5% of an alkali metal hydride, the alkali bath being used having a temperature of at least 185 C., the treatment being carried out so that the alkali penetrates only up to 50 to of the layer of scale. Thereafter the article is chilled down to room temperature by immersion into a water bath, washed free of alkali and subjected to a mechanical aftertreatment for removal of the loosely adhering scale,

Surprisingly it has been found that even when the scaly and rusty articles are subjected to a single run through of the process described, the scale is removed entirely even from very heavily scaled metal surfaces.

The process has the additional advantage that the caustic needs to penetrate the scale or rust layer only partially, with the result that the consumption of alkali hydride is correspondingly reduced as compared with the prior-art cleaning process. The scale or rust layer partially reduced by the caustic to the metallic or low oxide state adheres so loosely to the metal surface, after the chilling, that it can easily be removed by mechanical treatment, which is another advantage over the electrical post-treatment.

The molten caustic can be sodium hydroxide, potassium hydroxide or mixtures of the two.

The alkali metal hydride is preferably sodium hydride although potassium hydride and lithium hydride can also be used.

Different mechanical treatments are used, depending on the shape of the objects. For example, shaking apparatus are suitable for coils and spirals, while largesurface objects are cleaned off by water under pressure and compressed air or brushes.

The following examples are illustrative of the invention and are in no way to be construed as a limitation thereof.

EXAMPLE I A 350-kg. coil of rolled wire having a wire size of 6 mm. and a scale content of about 10 kg. per metric ton was immersed in a molten bath of NaOH/KOH with a hydride content of 0.9 weight percent calculated on NaOH/KOH at a temperature of 240 C. for 30 minutes with gentle swinging back and forth. Thereafter the coil was chilled in water down to room temperature and rinsed free of alkali,

The rinsed coil was then shaken on a shaking table, the loose layers flaking Off, the bright metal surface being thereby revealed.

'In a comparative experiment it was found by analysis that, after 2 hours of etching of the rolled wire used in the above example, the last increment of scale removed consisted substantially of iron.

The analysis of the last increment of scale removed after half an hour of etching as in the above example, however, had an iron content of only 71%.

EXAMPLE II The procedure of Example I was repeated. However following rinsing of the coil the same was subjected to jets of compressed air.

Substantially the same results were obtained.

What is claimed is:

1. In a process removing exceptionally heavy scale from an iron surface wherein said surface is immersed in a molten alkali metal bath containing 0.3 to 5 weight percent of an alkali metal hydride, the improvement comprising maintaining a bath temperature of at least C. and allowing alkali penetration to a depth of only 50 to 80 percent of the total thickness of the scale, thereafter chilling said surface to substantially room tem- 3 4 perature bycontact with water, washing said surface with I References Cited water to remove alkali and mechanically completing the UNITED STATES ATENTS removal of the scale.

2- -The process according to claim 1 wherein said gg g 1 m0lt n alkali bath c mprises at least one member selected 5 2717845 9/1955 g 52 2 fr m the group consisting of sodium hydroxide and 0- 2796366 6/1957 Carter 134 2 tassium hydroxide.

3. The process according to claim 1 wherein said alkali MORRIS O. WOLK, Primary Examiner metal hydride is Sodium hydride S. MARANTZ, Assistant Examiner 4. The process according to claim 1 wherein said 10 loosely adhering scale or rust is mechanically removed US. Cl. X.R.

by shaking. l341 

